When a constant current of an output current from a current source is realized in an electronic circuit, a constant current circuit is used. As the constant current circuit, a circuit using a metal-oxide-semiconductor field-effect transistor (MOSFET) is used. As the constant current circuit using the MOSFET, two examples will be mainly described below.
A first constant current circuit uses a source-grounded MOSFET. A gate-to-source voltage of the source-grounded MOSFET is fixed, and the MOSFET operates in a saturation region. Through this processing, the first constant current circuit is used as a constant current circuit.
A second constant current circuit is a cascode current source using a source-grounded MOSFET and a gate-grounded MOSFET.
As can be seen from the above discussion, two main constant current circuits are used, and each of the constant current circuits has the following defects.
In the first constant current circuit, the MOSFET operates in the saturation region. The constant current circuit exerts a channel length modulation effect in the saturation region. Due to the above channel length modulation effect, as an output voltage becomes higher, an output current becomes larger. As a result, there is a problem that output voltage dependence of the output current is large (output resistance is small).
In the second constant current circuit, when two MOSFETs stacked in two stages operate in the saturation region, output voltage dependence of the output current can be reduced (output resistance can be enlarged). Note that when both of two MOSFETs operate in the saturation region, a high output voltage is necessary as compared with a case of one MOSFET. Therefore, there is the following problem. That is, although the output voltage dependence of the output current can be reduced in the second constant current circuit, a lower limit of an operable output voltage range of a constant current circuit becomes high as compared with a first conventional example.
As can be seen from the above discussion, the two constant current circuits each have defects. For the purpose of removing the above defects, various constant current circuits are considered. For example, there is used a constant current drive circuit in which when a variable resistor arranged in a predetermined position operates in conjunction with the output voltage, even if an external load changes, an output current fails to change and a stabilized constant current can be provided also in the saturation region with a low voltage.    Japanese Laid-open Patent Publication No. 09-319323    Japanese Laid-open Patent Publication No. 2005-234890    Japanese Laid-open Patent Publication No. 2006-140888    Japanese Laid-open Patent Publication No. 2007-280322
However, in the conventional technology, when voltage dependence of the output current is reduced and a constant current circuit is operable also at the low output voltage, a circuit configuration of the constant current circuit becomes complicated. In the constant current drive circuit in which a variable resistor arranged in the predetermined position operates in conjunction with the output voltage, for example, another constant current source is necessary. In addition, a control circuit which gears the variable resistor to the output voltage is necessary and the entire circuit becomes complicated.